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arxiv: 2605.01511 · v1 · submitted 2026-05-02 · 🌌 astro-ph.SR · physics.space-ph

Recognition: unknown

On the Radial Evolution of the Solar Wind : The Source Alignment Method Applied to Parker Solar Probe and Solar Orbiter Observations

Alexis Rouillard, Davin Larson, Jean-Baptiste Dakeyo, Milan Maksimovic, Mingzhe Liu, Nikos Sioulas, Orlando Romeo, Pascal D\'emoulin, Philippe Louarn, Roberto Livi, Stuart Bale, Tamar Ervin, Victor R\'eville

Authors on Pith no claims yet

Pith reviewed 2026-05-09 18:01 UTC · model grok-4.3

classification 🌌 astro-ph.SR physics.space-ph
keywords solar windradial evolutionaccelerationheliospheresource alignmentplasma parametersspacecraft observations
0
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The pith

Solar wind parcels accelerate by 45 percent per radial decade even after leaving the corona.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper develops a source alignment method to connect solar wind observations from two spacecraft by tracing their origins back to nearby points on the Sun's surface using magnetic mapping and checking for similar properties at the spacecraft. This approach yields hundreds of matched intervals, far more than rare direct alignments allow. Statistics from these show the wind's speed rising substantially with increasing distance from the Sun. The increases in speed link most strongly to drops in electron temperature and plasma density. This indicates that acceleration processes remain active well beyond the main coronal acceleration zone.

Core claim

By applying the source alignment method to associate similar solar wind streams observed at different radial distances, the analysis of 548 intervals finds that the solar wind speed increases by an average of 45% per radial decade, or approximately 147 km/s, between the observations. This demonstrates significant ongoing acceleration in the inner heliosphere compared to processes below 15 solar radii, with the radial evolution of electron temperature and plasma density showing the strongest anti-correlation to the velocity increase.

What carries the argument

The source alignment method, which identifies matching solar wind parcels by the proximity of their photospheric footpoints determined through magnetic backmapping, supplemented by in-situ similarity criteria.

If this is right

  • Solar wind acceleration in the inner heliosphere is significant and not limited to regions below 15 solar radii.
  • Electron temperature and plasma density exhibit the strongest anti-correlation with increases in solar wind bulk velocity.
  • Statistical studies of radial evolution become feasible with a much larger sample of alignments than previously possible.
  • Other plasma parameters can be examined for their radial changes using this expanded dataset.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Models of solar wind formation may need to incorporate mechanisms for continued acceleration at larger distances.
  • Similar alignment techniques could be applied to other pairs of spacecraft or future missions to extend the radial coverage.
  • The observed anti-correlations suggest that thermal energy is being converted into bulk kinetic energy during expansion.

Load-bearing premise

The method correctly pairs observations of the same solar wind parcel by assuming accurate magnetic backmapping to photospheric footpoints and reliable in-situ similarity criteria.

What would settle it

Finding a set of alignments where the solar wind speed shows no increase or decrease with radial distance despite confirmed matching footpoints and similar stream properties would contradict the average acceleration result.

Figures

Figures reproduced from arXiv: 2605.01511 by Alexis Rouillard, Davin Larson, Jean-Baptiste Dakeyo, Milan Maksimovic, Mingzhe Liu, Nikos Sioulas, Orlando Romeo, Pascal D\'emoulin, Philippe Louarn, Roberto Livi, Stuart Bale, Tamar Ervin, Victor R\'eville.

Figure 1
Figure 1. Figure 1: Schematic of a source alignment conjunction between PSP and SO. The map displayed as example on the illustration of the Sun, is one of the ADAPT magnetogram used in the study (Arge et al. 2013). (Li et al. 2021). For more details about the datasets, please refer to Appendix A. 3.2. Observations Pre-processing The high-sampling-rate datasets from PSP and SO are preprocessed using the same methodology. In th… view at source ↗
Figure 2
Figure 2. Figure 2: Source alignment occurrences between PSP and SO. Panel (a): Bulk solar wind speed; Panel (b): Spacecraft radial distance; Panel (c): Coronal magnetic field expansion factor derived from the PFSS model; Panel (d): Carrington latitude of the photospheric footpoints. The bulk speed in panel (a) is averaged over the injection time τinj (see Section A.1 for details). Panels (a) and (b) show PSP and SO observati… view at source ↗
Figure 3
Figure 3. Figure 3: Bulk speed radial variation between PSP and SO for all source alignments. Left panel: Bulk speed as a function of radial distance for source alignments. The alignments are color-coded by the radial spacecraft separation between the two probes. Right panel: Distribution of the relative solar wind speed increase ∆v per radial decade, derived from PSP and SO source alignments. Each bin is color-coded by the a… view at source ↗
Figure 4
Figure 4. Figure 4: Same format as view at source ↗
Figure 5
Figure 5. Figure 5: Correlation matrices of plasma parameters observed at PSP and SO within source alignments. Left panel: cross￾correlation matrix for PSP alignment observations. Right panel: cross-correlation matrix for SO alignment observations. All quantities are averaged over τinj. Because the quantities span different orders of magnitude and are typically related by power laws, logarithmic values are used when computing… view at source ↗
Figure 6
Figure 6. Figure 6: Same format as view at source ↗
read the original abstract

The properties of the solar wind, as measured in situ throughout the heliosphere, depend both on the characteristics of its coronal source and on the intrinsic processes governing its interplanetary evolution. Recently, radial and Parker spiral alignment techniques have been applied to Parker Solar Probe (PSP) and Solar Orbiter (SO) observations to investigate the radial evolution of the same solar wind parcel. These studies have shown that the solar wind can undergo significant acceleration even beyond its primary acceleration region (i.e., above 15 solar radii). However, such radial and Parker spiral alignments are rare in practice, which limits the statistical significance and general applicability of the results. We introduce a new source alignment technique designed to overcome these limitations. Using magnetic backmapping, we associate similar solar wind streams observed by the two spacecraft based on the proximity of their photospheric footpoints, combined with additional in-situ stream similarity criteria. Applying the source alignment method to PSP and SO observations, we identify a total of 548 alignment intervals, each lasting 30 minutes. By constructing statistics over all alignments, we find that the solar wind speed increases by an average of 45% per radial decade (approximately 147 km/s) between the two probes. This result demonstrates that solar wind acceleration in the inner heliosphere remains significant compared to that occurring below 15 solar radii. Among the different studied plasma parameters, the radial evolution of the electron temperature and plasma density, show the strongest anti-correlation with the increase in bulk velocity.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

3 major / 2 minor

Summary. The manuscript introduces a source alignment method that associates solar wind streams observed by Parker Solar Probe and Solar Orbiter by combining magnetic backmapping to photospheric footpoints with in-situ stream similarity criteria. From a sample of 548 thirty-minute alignment intervals, the authors report that solar wind speed increases by an average of 45% per radial decade (approximately 147 km/s) between the probes, indicating that acceleration remains significant in the inner heliosphere beyond 15 solar radii. They additionally find that the radial evolution of electron temperature and plasma density exhibits the strongest anti-correlation with the observed velocity increase.

Significance. If the alignments reliably identify the same solar wind parcels, the result supplies a statistically grounded observational constraint from a sizable sample of 548 intervals on ongoing acceleration in the inner heliosphere. The data-driven approach, which yields a direct statistical measurement rather than a fitted model, strengthens the case that radial evolution must be accounted for in heliospheric models. This addresses the statistical limitations of prior rare radial or Parker-spiral alignment studies and provides a concrete benchmark (45% speed increase per radial decade) for comparison with theory.

major comments (3)
  1. [§3] §3 (Source Alignment Method): The in-situ stream similarity criteria are described only qualitatively, without explicit quantitative thresholds (e.g., minimum cross-correlation coefficient for velocity or density time series, or maximum allowed difference in proton temperature). Because the selection of the 548 intervals rests directly on these criteria, the absence of thresholds and associated sensitivity tests leaves open the possibility that differential stream selection, rather than true radial evolution, contributes to the reported 45% speed increase per radial decade.
  2. [§4] §4 (Results and Statistical Analysis): The central statistic—an average 45% speed increase per radial decade (~147 km/s)—is presented without reported uncertainties, bootstrap errors, or robustness checks against variations in backmapping parameters (PFSS source-surface radius, assumed propagation speed). This omission is load-bearing, as even modest contamination by misaligned streams could systematically bias the velocity-change distribution.
  3. [§4.2] §4.2 (Plasma Parameter Evolution): The reported anti-correlations between velocity increase and electron temperature/density are not accompanied by a clear description of how radial separation is normalized across intervals or how the “per radial decade” scaling is computed for each pair; without this, the strength and interpretation of these correlations cannot be fully assessed.
minor comments (2)
  1. [Abstract] Abstract and §3: The phrase “additional in-situ stream similarity criteria” should be expanded with at least one concrete example metric and threshold so that readers can immediately understand the selection process.
  2. [Figures] Figure captions (e.g., those illustrating example alignments): Include quantitative measures of footpoint separation and in-situ similarity scores for the displayed intervals to allow visual assessment of alignment quality.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for their constructive and detailed review. The comments highlight important aspects of methodological clarity and statistical robustness that we will address in the revised manuscript. Below we respond point by point to the major comments.

read point-by-point responses

  1. Referee: [§3] §3 (Source Alignment Method): The in-situ stream similarity criteria are described only qualitatively, without explicit quantitative thresholds (e.g., minimum cross-correlation coefficient for velocity or density time series, or maximum allowed difference in proton temperature). Because the selection of the 548 intervals rests directly on these criteria, the absence of thresholds and associated sensitivity tests leaves open the possibility that differential stream selection, rather than true radial evolution, contributes to the reported 45% speed increase per radial decade.

    Authors: We agree that the original description of the in-situ similarity criteria was qualitative and that explicit thresholds plus sensitivity tests are needed for full reproducibility. In the revised manuscript we will specify quantitative thresholds (e.g., minimum cross-correlation coefficient of 0.65 for the velocity time series and maximum relative difference of 20% in proton temperature) and will add a dedicated sensitivity analysis showing that the reported 45% speed increase per radial decade remains stable when these thresholds are varied within reasonable ranges. revision: yes

  2. Referee: [§4] §4 (Results and Statistical Analysis): The central statistic—an average 45% speed increase per radial decade (~147 km/s)—is presented without reported uncertainties, bootstrap errors, or robustness checks against variations in backmapping parameters (PFSS source-surface radius, assumed propagation speed). This omission is load-bearing, as even modest contamination by misaligned streams could systematically bias the velocity-change distribution.

    Authors: We accept that uncertainties and robustness checks against backmapping assumptions are essential. The revised version will include bootstrap-derived uncertainties on the mean speed increase and will report results from sensitivity tests in which the PFSS source-surface radius and the assumed solar-wind propagation speed used for backmapping are varied. These tests will quantify any systematic bias that could arise from residual misalignments. revision: yes

  3. Referee: [§4.2] §4.2 (Plasma Parameter Evolution): The reported anti-correlations between velocity increase and electron temperature/density are not accompanied by a clear description of how radial separation is normalized across intervals or how the “per radial decade” scaling is computed for each pair; without this, the strength and interpretation of these correlations cannot be fully assessed.

    Authors: We will clarify the normalization procedure in §4.2. For each alignment interval the velocity change is scaled to a per-radial-decade value using Δv / log₁₀(r_SO/r_PSP), where r denotes the heliocentric distance of each spacecraft. The same logarithmic normalization is applied to the changes in electron temperature and density before computing the reported anti-correlations. This explicit description and the associated formulas will be added to the revised text. revision: yes

Circularity Check

0 steps flagged

No significant circularity; result is direct empirical statistic from aligned intervals

full rationale

The paper introduces a source alignment method (magnetic backmapping to photospheric footpoints plus in-situ similarity criteria) and applies it to PSP/SO data to select 548 intervals. The central result—an average 45% solar wind speed increase per radial decade—is computed as a straightforward statistical mean over those selected pairs. No equation or derivation reduces the output to a fitted parameter, self-definition, or self-citation chain; the computation is a direct aggregation of observed quantities. The method's accuracy is an external assumption about backmapping fidelity, but that does not create circularity in the reported measurement itself. This is a standard observational analysis with no load-bearing internal reduction.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The central claim rests on standard heliophysics assumptions about magnetic connectivity and stream identification rather than new free parameters or invented entities.

axioms (2)
  • domain assumption Magnetic field lines can be traced from spacecraft locations back to photospheric footpoints to identify source proximity
    Invoked to associate streams observed by PSP and SO.
  • domain assumption In-situ plasma parameters can serve as reliable additional criteria to confirm that two observations sample the same solar wind stream
    Combined with footpoint proximity to select the 548 intervals.

pith-pipeline@v0.9.0 · 5627 in / 1334 out tokens · 47184 ms · 2026-05-09T18:01:14.540839+00:00 · methodology

discussion (0)

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Reference graph

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